This invention relates to a method for the induction hardening of electrically and thermally conducting workpieces and more particularly to a method for the induction hardening of such electrically and thermally conducting workpieces substantially similar to a gear in shape. This method uses a single pulse of alternating current passed through the inductor surrounding the workpiece without the necessity for preheating the workpiece. This method uses only a single frequency of alternating current to accomplish contour hardening of the workpiece, including hardening of tooth and root regions of the workpiece, while avoiding through-tooth hardening with its attendant damage in gear performance.
In the manufacture of gears, it has been standard practice in the industry for several decades to harden the tooth and root regions of the gear to increase wear resistance and thereby to improve the performance and lifetime of the gear. The methods by which this hardening of the workpiece have been accomplished have evolved considerably over the years, reflecting the continuous need to increase processing speed, reliability and performance of the workpiece, while reducing manufacturing costs.
The conventional hardening method involves carburizing the outer surface of the gear, including the tooth and root regions, while typically shielding the remainder of the gear from carburization. In a typical process, the gear would be coated with a thin layer of copper over all regions except for the tooth and root regions intended to be carburized. Following copper coating, the entire gear would typically be heated in a furnace in a carburizing atmosphere, exposing the entire gear to the surrounding carburizing medium. The carburization would occur in the unplated tooth and root regions, while the copper coated portions of the gear would remain free from carburization. The copper coating would then be chemically removed in the final step of this carburizing process.
The carburization process as typically performed involves many processing steps. These naturally increase processing costs and reduce the number of gears which can be processed in a given amount of time. Also, the heating of the entire gear in a furnace typically induces significant distortion of the shape and dimensions of the gear. In many instances, these distortions are sufficient to cause the gear no longer to meet specifications, requiring subsequent machining to restore the gear to acceptable form. This machining step clearly adds to the time and cost of the finished gear. In addition, in order to bring the gear into its final shape, this machining process must be performed on the fully hardened gear. In many cases the machining of the hardened gear causes severe wear on the machining tools themselves, further adding to the manufacturing costs.
For many of the reasons described above, induction hardening was investigated as early as the 1940's as a method for case hardening gears in a more effective manner. Induction hardening involves passing alternating current through an inductor held in close proximity to the part to be hardened. The current-carrying inductor is surrounded by an alternating magnetic field which induces currents in the nearby electrically and thermally conducting workpiece (the gear in the present instance). The workpiece is thus heated by resistive heating caused by the induced currents. Regions of the workpiece too distant from the inductor will not be directly heated by induced currents. However, these regions may experience sufficient heating by means of thermal conduction from adjacent regions to produce the desired metallurgical properties.
The heating of the workpiece is affected by many factors, including: the frequency of the current, its magnitude and duration, the proximity of the coil to the workpiece, the configuration of the inductor coil, and the timing of heating and quenching, to name a few. However, the relative ease of heating by induction makes it a very attractive process in use today in many factories for the hardening of metals.
However, the hardening of gears by induction presents certain problems of its own. Typically, it is desired to harden both the tooth and the root regions of the gear. Therefore, both regions must be brought to a temperature exceeding the transfomation temperature. The penetration depth of induced currents in induction hardening process (hence the depth of heating, or case depth) varies inversely with the frequency of the alternating current. Thus, relatively low frequency current (typically 3-10 KHz) will heat effectively the root region of the gear, but not the tooth. Conversely, much higher frequency currents (typically 100-400 KHz, loosely called "radiofrequency" or "RF") heat the tooth of the gear quite efficiently, but such currents do not penetrate to heat the root region except by thermal conduction from the heated tooth. Much of the work on hardening of gears by induction heating has involved various methods to deal with the problems of case hardening both root and tooth regions as economically as possible.
One approach to this problem has been considered as early as 1950 in the report of J. A. Redmond ("Heat Treating Gears by Induction Heating", 34th Annual Meeting of the American Gear Manufacturers Association, June 1950). He notes in this report that 10 KHz induction heating can be used exclusively to harden gears if the heat treating is carried out to such an extent that the tooth region is heated by thermal conduction from the root. The 10 KHz induction current heats the root region most effectively by inducing currents completely encircling the circumference of the gear in the region of the root. If sufficient heat is generated for sufficient time, the tooth region of the gear will also be hardened by conduction heating from the induction-heated root region. However, as noted by Redmond, this technique has the drawback of heating the entire tooth of the gear completely through from one flank to the other. This causes the tooth to become more brittle and may break off from the gear when the gear is subjected to vigorous use. "Contour hardening" of just the surface regions of the tooth, root and flanks of the gear is desired to produce the desired resistance to wear without introducing structural brittleness into the gear.
One approach to the problem of contour hardening of gears without through-tooth hardening is to harden the gear one tooth at a time. Typically, an inductor coil is used having the approximate shape of the region between adjacent gear teeth and able to pass between adjacent teeth in close proximity to both. Such an inductor is commonly called an "intensifier". The intensifier is passed between adjacent gear teeth, hardening root and flanks as it does so. (Typically, such a method does not harden the very uppermost tip of the tooth, but this region is not often subject to serious wear and lack of hardening is not considered an important practical problem.) In operation, many gears are arranged in a stack such that a single pass of the intensifier heats a single root and flank region of many gears. Even so, the speed of the process is not favorable for high volume production and the part handling equipment tends to be complex.
Another approach to the effective contour hardening of gears has been a two-step process involving a preheating of the gear followed by a second heating step for the final hardening. Typically, two frequencies of induction current are used. A first preheating current is used (typically in the frequency range of 1 KHz to 10 KHz) which heats primarily the root region of the gear and, by thermal conduction, also heats the immediate surroundings. Following this preheating step (or sequence of steps), a final heating step is typically performed by means of a high frequency RF current to harden the tooth region. By means of this multistep process, the root and tooth regions can be heated to the proper temperature for the proper duration, without overheating the tooth region resulting in through-tooth hardening and brittleness.
This two-frequency process was described as early as 1950 in the reference by Redmond cited above and is still in active use. (As in the recent work by J. M. Storm and M. R. Chaplin, "Dual Frequency Induction Gear Hardening", American Gear Manufacturers Association, 1986). A recent patent to Mucha et. al. U.S. Pat. No. 4,675,488) describes a two-frequency process in which two preheating steps employing relatively low frequency current are used followed by a single final heating with RF current.
The dual frequency induction hardening of gears offers definite advantages over older methods in providing a relatively simple and rapid gear hardening process. However, certain problem remain. As illustrated in the Mucha patent, two separate induction heating coils are employed (typically, but not always, connected to two separate sources of alternating current). The gear to be hardened must be physically moved from one inductor coil to the other, increasing the complexity of the process and reducing speed. Also, as typified in the patent of Mucha, multiple heating steps may be required at each frequency (hence, in each inductor coil). This further complicates the problems of equipment and process control.
A very significant disadvantage of multistep induction heat treating processes is the possibility that later heating steps may reduce (or completely negate) the benefit done by previous steps in the gear manufacturing process. Typically, a gear is machined from unhardened steel and then subjected to a furnace heat treating operation to improve the toughness of the overall gear structure without introducing brittleness. Steel hardened by such "quench and temper" operations do not typically have the surface wear characteristics required of the final part. Multistep induction heat treating, intended to provide the desired wear characteristics, runs the risk of destroying wholly or partially the beneficial effects of the quench and temper.
Typically, a first heating step (or steps) in a multistep surface hardening procedure will be employed to pre-heat the root region of a gear. This is typically followed by a second heating step (or steps) for the purpose of hardening the tooth region while, at the same time, introducing sufficient additional heat into the root region to achieve hardening there also. However, the second heating step may add sufficient heat to the gear to temper back the previously achieved microstructure by thermal conduction. This results in a reduction of microhardness in a transitional manner from tooth to root.
All multistep heating processes run the risk of undoing by later heating any good done by prior heating. This possibility makes process control very important, further increasing processing costs. A major goal of the present invention is to use a single-step heating process to harden both tooth and root regions, thereby avoiding this potential problem
In recognition of some of these problems, recent work by Chatterjee (U.S. Pat. No. 4,639,279) has eliminated the need for dual frequency heating. The process described by Chatterjee uses a single frequency (typically in the intermediate range of 50-100 KHz) for both preheating and final heating steps. This involves the use of only a single inductor coil and does not require the workpiece to be physically moved from place to place during the induction hardening process. This represents an important simplification of the induction hardening process for gears.
However, even with the recent developments of Chatterjee, certain problems remain which is the intent of the present invention to address. The Chatterjee invention is still a multistep heating process, although using a single frequency of induction heating current for each step. Thus, the problem still remains of controlling later steps sufficiently carefully to avoid undoing part or all of the benefits of earlier processing.
The developments of Chatterjee require a preheating phase at much lower power levels (but at the same frequency) as the final heating of the workpiece. This increases the time that the workpiece is exposed to heat over the rapid, single-step process disclosed in the present invention. Thus, the distortion of the workpiece is expected to be much lower for the present invention, reducing the need for later reshaping of the workpiece.
In addition, according to his disclosure, the invention of Chatterjee requires rather careful control of the rate at which the maximum power level is obtained (the "ramping"). It is an object of the present invention to describe an improved method for the induction hardening of gears in which no preheating is required, and precise control of the power ramping in this single heating step is likewise not required. It is submitted that these simplifications in the process represent important practical improvements in the art of induction contour hardening of gears.